Title:
SCALING LAWS FOR DARK MATTER HALOS IN LATE-TYPE AND DWARF SPHEROIDAL GALAXIES
John Kormendy
Department of Astronomy, The University of Texas at Austin
Abstract:
Published mass models fitted to galaxy rotation curves are used to study
the systematic properties of dark matter (DM) halos in late-type and dwarf
spheroidal (dSph) galaxies. Halo parameters are derived by fitting non-singular
isothermals to the part of the rotation curve that is not explained by visible
matter. Rotation curve decomposition becomes impossible fainter than absolute
magnitude M_B = -13, where the rotation velocity becomes comparable to the
velocity dispersion of the gas. To increase the luminosity range further,
we include dSph galaxies, which are physically related to spiral and irregular
galaxies. Combining the data, we find that DM halos satisfy well
defined scaling laws analogous to the "fundamental plane" relations for
elliptical galaxies. Halos in less luminous galaxies have smaller core radii,
higher central densities, and smaller central velocity dispersions. Scaling
laws provide new and detailed constraints on the nature of DM and on galaxy
formation and evolution. Some simple implications include:
1 -- A single, continuous physical sequence of increasing mass extends
from dSph galaxies with M_B = -7.6 to Sc I galaxies with M_B = -22.4.
2 -- The high DM densities in dSph galaxies are normal for such tiny
galaxies. Since virialized density depends on collapse redshift
z_coll -- density is proportional to (1 + z_coll) cubed -- the smallest
dwarfs formed at least redshift 7 earlier than the biggest spirals.
3 -- The high DM densities of dSphs implies that they are real galaxies
formed from primordial density fluctuations. They are not tidal fragments.
Tidal dwarfs cannot retain even the low DM densities of their giant-galaxy
progenitors. In contrast, dSphs have higher DM densities than do giant-galaxy
progenitors.
4 -- The fact that, as luminosity decreases, dwarf galaxies become much
more numerous and also more nearly dominated by DM raises the possibility that
there exists a large population of objects that are completely dark. Such
objects are a canonical prediction of cold DM theory. If they exist, ``empty
halos'' are likely to be small and dense -- that is, darker versions of Draco
and UMi.
5 -- The slopes of the DM parameter correlations provide a measure on
galactic mass scales of the slope n of the power spectrum of primordial
density fluctuations. Our preliminary results not yet corrected for baryonic
compression of DM give n = -1.9 plus or minus 0.2. This is consistent with
the theory of cold dark matter.